1. Field of the Invention
The present invention relates to pseudo halftone processing for representing a multi-gradation image in two gradations. Particularly, the present invention relates to a dither pattern generation method that is used when executing a quantization process by a dithering method.
2. Description of the Related Art
In pseudo halftone processing that represents a multi-gradation image in two gradations: (1) dot printing and (0) no dot printing, performing quantization processing is desired in order to convert multi-value density data to two values. As typical quantization processing methods there is the error diffusion method or the dither method, however, from the aspect of being able to achieve quantization at high speed with a small processing load, the dither method is particularly used in high-speed printers for which speed is required, or in multi-color printers for which there is much data.
The dot arrangement on a printing medium is determined according to this kind of quantization processing, and particularly in low-gradation areas from highlighted areas, high dispersibility is required in order to suppress a visual granular feeling. Therefore, in both the error diffusion method and dither method, various methods for increasing dispersibility of dots in low-gradation areas have been proposed. However, even though quantization processing having this kind of high dispersibility may be employed, there may be times, such as when printing a color image, in which the granular feeling becomes worse due to overlapping of plural dots on the surface of the paper. For example, when absolutely the same quantization processing is employed for each color, dots of all colors are printed in the same locations, and the granular feeling increases. Moreover, in cases in which quantization processing is performed for each color independently and with no correlation, there is a possibility that as a result of overlapping there will be dot sparseness and denseness of dots and that dispersibility will be lost even though there is excellent dispersibility for each individual color.
In regards to such problems, Japanese Patent No. 4574470 discloses a method for generating dither patterns so that dispersibility that results from superimposing dot arrangements of plural colors will become a maximum. By using the dither patterns disclosed in Japanese Patent No. 4574470, it is possible to output an image having high dispersibility and in which a granular feeling has been suppressed, and at the same time avoid beading (clumping together of dots) even for low-gradation areas of a color image.
However, in Japanese Patent No. 4574470, the dispersibility that results from superimposing the dot arrangements for plural colors is taken into consideration when setting the dither patterns for each color, so in the case of printing with only one color, when compared to a conventional dither pattern that is generated taking into consideration only one color, there were cases in which dispersibility decreased. In other words, conventionally, it was difficult to generate a dither pattern that could achieve high dispersibility for both the case of a single color and the case of mixed colors.